Method for cleaning a solar cell surface opening made with a solar etch paste

ABSTRACT

A thin silicon solar cell having a back dielectric passivation and rear contact with local back surface field is described. Specifically, the solar cell may be fabricated from a crystalline silicon wafer having a thickness from 50 to 500 micrometers. A barrier layer and a dielectric layer are applied at least to the back surface of the silicon wafer to protect the silicon wafer from deformation when the rear contact is formed. At least one opening is made to the dielectric layer. An aluminum contact that provides a back surface field is formed in the opening and on the dielectric layer. The aluminum contact may be applied by screen printing an aluminum paste having from one to 12 atomic percent silicon and then applying a heat treatment at 750 degrees Celsius.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of U.S.Provisional Patent Application No. 60/916,327, filed May 7, 2007.

GOVERNMENT INTERESTS

The U.S. Government has a paid-up nonexclusive, worldwide license inthis invention and the right in limited circumstances to require thepatent owner to license others on reasonable terms as provided for bythe terms of contract No. DE-FC36-07GO17023 awarded by the U.S.Department of Energy.

FIELD OF THE INVENTION

The present invention generally relates to silicon solar cells. Moreparticularly, the present invention relates to a formation of a back orrear contact that provides back surface passivation and opticalconfinement properties.

BACKGROUND OF THE INVENTION

Solar cells are devices that convert light energy into electricalenergy. These devices are also often called photovoltaic (PV) cells.Solar cells are manufactured from a wide variety of semiconductors. Onecommon semiconductor material is crystalline silicon.

Solar cells have three main elements: (1) a semiconductor; (2) asemiconductor junction; and (3) conductive contacts. Semiconductors suchas silicon may be doped n-type or p-type. If an n-type silicon andp-type silicon are formed in contact with one another, the region in thesolar cell where they meet is a semiconductor junction. Thesemiconductor absorbs light. The energy from the light may betransferred to the valence electron of an atom in a silicon layer, whichallows the valence electron to escape its bound state leaving behind ahole. These photogenerated electrons and holes are separated by theelectric field associated with the p-n junction. The conductive contactsallow current to flow from the solar cell to an external circuit.

FIG. 1 shows the basic elements of a prior art solar cell. The solarcells can be fabricated on a silicon wafer. The solar cell 5 comprises ap-type silicon base 10, an n-type silicon emitter 20, bottom conductivecontact 40, and a top conductive contact 50. The p-type silicon base 10and the n-type silicon emitter 20 contact one together to form thejunction. The n-type silicon 20 is coupled to the top conductive contact50. The p-type silicon 10 is coupled to the bottom conductive contact40. The top conductive contact 50 and the bottom conductive contact 40are coupled to a load 75 to provide it with electricity.

The top conductive contact 50 (“front contact”), comprising silver,enables electric current to flow into the solar cell 5. The topconductive contact 50, however, does not cover the entire face of thecell 5 because silver is not entirely transparent to light. Thus, thetop conductive contact 50 has a grid pattern to allow light to enterinto the solar cell 5. Electrons flow from the top conductive contact50, and through the load 75, before uniting with holes via the bottomconductive contact 40.

The bottom conductive contact 40 (“rear contact” or “back contact”)usually comprises aluminum-silicon eutectic. This conductive contact 40typically covers the entire bottom of the p-type silicon 10 in order tomaximize conduction. The aluminum is alloyed with silicon at hightemperatures of approximately 750 degrees Celsius, well above thealuminum-silicon eutectic temperature of 577 degrees Celsius. Thisalloying reaction creates a heavily-doped p-type region at the bottom ofthe base and gives rise to a strong electric field there. This fieldaids in repelling the light-generated electrons from recombining withholes at the back contact so that they can be collected more efficientlyat the p-n junction.

The interface between silicon and a conductive contact is typically anarea having high recombination. For example, the back surfacerecombination velocity of an aluminum back surface field across theentire back surface may be 500 centimeters per second or more. High backsurface recombination velocities decrease cell efficiency.

SUMMARY OF THE INVENTION

One method that has been used to reduce recombination at the backcontact is to form a dielectric layer of silicon dioxide on the rearsurface of the silicon wafer. This dielectric layer improvespassivation, but creates other problems such as how to generate openingsfrom the dielectric layer to the silicon, and optimizing the size andspacing of each window. In addition, the dielectric layer does notprotect the silicon wafer from aluminum-silicon alloying during contactformation, which may deform the silicon wafer. Thin film silicon wafersare especially susceptible to deformation. The prior art solutions forreducing recombination at the back surface do not adequately addressother issues such as preventing thin film silicon deformation,determining the size and spacing of dielectric openings, cleaning thedielectric openings, and forming quality back surface fields at thedielectric openings.

The solution as presented herein comprises a solar cell structure thathas a dielectric passivation layer and a rear contact with localaluminum back surface field. A process for forming the rear contact isprovided. In an embodiment, a dielectric layer is formed on the rearsurface of a thin crystalline wafer having an n-region and a p-region.An opening is made in the dielectric layer by screen printing an etchpaste, followed by a first heat treatment. A hydrofluoric acid solutionmay be used to remove any residue left by the etch paste. The rearcontact is formed by screen printing a contact paste on the entire backsurface followed by a second heat treatment. The contact paste iscomprised of aluminum and from one to 12 atomic percent silicon. Thepresence of the silicon in the contact paste saturates the appetite ofaluminum for silicon during the second heat treatment, and provides ahigh-quality back surface field contact at the local openings. The useof little or no glass frit in the aluminum helps to avoid significantaluminum spiking through the dielectric layer which degrades deviceperformance.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, inventive features, and advantages of the present disclosure,as defined solely by the claims, will become apparent in thenon-limiting detailed description set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a prior art solar cell.

FIG. 2 is a flowchart for one embodiment of a process for forming a backcontact with local back surface field.

FIG. 3A is a DESSIS simulation domain for a line back contact.

FIG. 3B is a DESSIS simulation domain for a point back contact.

FIG. 4A is a DESSIS output graph that shows spacing versus efficiencyfor contacts having 75 micrometer width.

FIG. 4B is a DESSIS output graph that shows spacing versus efficiencyfor contacts having 150 micrometer width.

FIG. 5A to 5D are cross-sectional views from an electron microscope oflocal back surface fields for different aluminum contact pastes.

FIGS. 6A to 6E are cross-sectional views for one embodiment of a siliconwafer at each stage of the back contact fabrication process.

FIG. 7A is a bottom plan view for one embodiment of window openings tosilicon having a point pattern.

FIG. 7B is a bottom plan view for one embodiment of window openings tosilicon having a line pattern.

FIG. 8 is a top view from an electron microscope of an opening of adielectric layer exposed with a screen printing etch paste.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components, andcircuits have not been described in detail so as not to obscure thepresent invention.

FIG. 2 depicts a flowchart for forming a high quality rear contact thatprotects the silicon wafer from damage during the alloying process andprovides a local back surface field. A local back surface field (BSF) isdesirable because it helps to reduce the recombination of electrons atthe solar cell's back surface. Efficiency of the solar cell is therebyincreased if the solar cell has a high quality local BSF.

In operation 200, a p-type or n-type layer is formed on a silicon wafer.The silicon wafer may be crystalline. The silicon wafer may have athickness from 200 to 250 micrometers. For another embodiment, thesilicon wafer may have a thickness from 50 to 500 micrometers.Aluminum-silicon alloying across the entire back surface of the siliconwafer may deform thin silicon wafers. Thus, rather than forming fullarea contacts directly on the silicon wafer, barrier and dielectriclayers are grown on the front and back sides of the silicon wafer inoperation 210. The dielectric layers may be grown concurrently orsimultaneously. For one embodiment of the invention, the dielectriclayers are silicon dioxide. For another embodiment of the invention, thedielectric layers may be aluminum oxide.

Silicon dioxide may be formed through a spin-on process to achieve athickness from 1000 to 5000 angstroms on each side. During the spin-onprocess, the dielectric in liquid form is deposited onto spinningwafers. The spin-on precursor may be a silicon dioxide sol-gel. Silicondioxide sol-gel is commercially available from Filmtronics, Inc. underthe name “20B.” After the spin-on process, the wafer is dried at atemperature from 150 to 250 degrees Celsius for between 10 and 20minutes. The silicon dioxide may be cured in oxygen ambient at atemperature from 875 to 925 degrees Celsius in a conventional tubefurnace. The spin-on process enables a thicker, more uniform, silicondioxide layer to be formed, which makes the dielectric a diffusion maskfor single sided diffusion.

Alternatively the silicon dioxide may be formed via a chemical vapordeposition process or plasma enhanced chemical vapor deposition (PECVD)process. Such process may use silane and oxygen as precursors at atemperature from 300 to 500 degrees Celsius for 10 to 20 minutes. Areaction chamber may be used to control the reactants for this process.

In operation 215, a barrier layer is formed on the front and back sidesof the wafer. The barrier layer may be comprised of silicon nitridehaving a thickness from 100 to 700 angstroms. The silicon nitride layermay be formed using PECVD. Silane and ammonia may be PECVD precursors ofsilicon and nitride, respectively. Alternatively, the silicon nitridelayer may be formed using a low pressure chemical vapor depositionprocess in a suitable reaction chamber. The barrier layer on the frontsurface provides an anti-reflective coating to help absorb light. Thebarrier layers also protect the dielectric layers. Without the barrierlayer on the back surface, the back surface dielectric layer may besubjected to aluminum spiking and impurities through the air. Moreover,the dielectric layers are more vulnerable to damage by high temperatureduring the firing of the screen printed contacts without the barrierlayers.

In operation 220, at least one opening is formed in the dielectric andbarrier layers on the back side of the silicon wafer. If a plurality ofopenings are formed, the openings may be evenly distributed across thesurface of the silicon wafer. For one embodiment of the invention, theopening is made by applying a solar etch paste to the barrier layer. Anexemplary solar etch paste is that manufactured by Merck & Co., Inc.under the name “Solar Etch AX M1.” The solar etch paste may also be usedto make openings to the front surface dielectric layer. The etch pastemay comprise phosphoric acid, hydrofluoric acid, ammonium fluoride, orammonium hydrogen fluoride. The openings formed in operation 220 may bein the shape of points or lines.

The paste should only be applied to the areas where openings in thedielectric layer are desired. The paste may be applied using a screenprinting machine. The optimum size and spacing of the openings to thesubstrate are a function of the resistivity of the wafer. Computerprograms such as Device Simulations for Smart Integrated Systems(DESSIS) may be used to determine the optimum size and spacing of theopenings. DESSIS calculates optimum spacing based on parametersincluding contact type (point or line), contact size (75 micrometers or150 micrometers), and lateral BSF (presence or absence). The simulationdomain is derived from the smallest unit cell that can be extendedperiodically to represent the complete structure. To simplify thesimulation problem, front contact parameters may be defined such thatthe front contact is uniformly distributed. Under this scenario, thesize of the unit cell is controlled by the back contact geometry in theDESSIS simulation.

The simulation domain for a line contact is shown in FIG. 3A. Thesimulation domain of FIG. 3A comprises a p-type silicon 300, an n-typesilicon 310, a dielectric layer 320, a first conductive contact 330, asecond conductive contact 360, and a local BSF 370. The p-type silicon300 is coupled to n-type silicon 310, dielectric layer 320, and localBSF 370. The local BSF 370 is coupled to the second conductive contact360. The n-type silicon 310 is coupled to the first conductive contact330.

Similarly, a simulation domain for a point contact is shown in FIG. 3B.The simulation domain of FIG. 3B comprises a p-type silicon 300, ann-type silicon 310, a dielectric layer 320, a first conductive contact330, a second conductive contact 360, and a local BSF 370. The p-typesilicon 300 is coupled to n-type silicon 310, dielectric layer 320, andlocal BSF 370. The local BSF 370 is coupled to the second conductivecontact 360. The n-type silicon 310 is coupled to the first conductivecontact 330.

The optical generation parameters may be set to assume a uniform lightincident on a textured silicon surface having a facet angle of 54.7degrees, an antireflection layer of index 2.0, and a thickness of 75nanometers. The incident light may also be decreased by approximately8.5 percent to account for shading by a front contact in the actualdevices. The internal front surface reflection may be set to 92 percent.The back surface reflection may be set to 85 percent.

The emitter profile may be a Gaussian profile with a peak n-type dopingconcentration at the surface of 1.14×10²⁰ per cubic centimeter and ajunction depth of 0.3 micrometers, which correspond to an emitter havinga sheet resistance of approximately 80 ohms per square. Alternatively,an emitter sheet resistance may be varied from 70 to 90 ohms per square.

The local BSF at the back contact may be defined to have a constantp-type doping concentration of 1×10¹⁹ per cubic centimeter with athickness of 1.47 micrometers. This results in an effective surfacerecombination velocity of approximately 300 centimeters per second atthe contact on a 2.0 ohm-centimeter substrate. To simulate for lateralBSF, the BSF layer may be extended laterally to at least 1.3 micrometersoutside the contact edge. To simulate for no lateral BSF, the BSF layermay be defined to only cover the contact area.

Other parameter settings may include a cell thickness from 50 to 200micrometers, a resistivity from 1.5 to 2.5 ohm-centimeter, a frontsurface recombination velocity from 50,000 to 70,000 centimeters persecond, a back surface recombination velocity at the dielectric from 40to 60 centimeters per second, and a contact resistance of zeroohm-centimeter squared. Using these parameters, a DESSIS output graphdepicting solar cell efficiency depending on contact spacing forcontacts having a 75 micrometer width is shown in FIG. 4A, and a graphdepicting solar cell efficiency depending on contact spacing forcontacts having a 150 micrometer width is shown in FIG. 4B.

After applying the etch paste, the etch paste is exposed to a heatsource at a temperature from 300 to 380 degrees Celsius for 30 to 45seconds. The heat source coupled with the solar etch paste dissolves thebarrier layer and the dielectric layer under the paste leaving anopening to the substrate. A hydrofluoric acid solution may be used toremove any resulting residue in or around the opening.

For another embodiment of the invention, the openings in the dielectriclayer may be made using a laser or a mechanical scribe. The openings maycover one to 10 percent of the rear surface area. The dielectric layerremains on the remainder of the rear surface following operation 220.

In operation 230, a rear contact layer is applied with an aluminum pastethat contains from one to 12 atomic percent silicon. For one embodimentof the invention, the aluminum paste may be product number: AL 53-090,AL 53-110, AL 53-120, AL 53-130, AL 53-131, or AL 5540 which are allcommercially available from Ferro Corporation. For another embodiment ofthe invention, the aluminum paste may be commercially available aluminumpaste manufactured by DuPont Corporation, Cermet Materials, Inc., ChimetChemicals, Cixi Lvhuan Healthy Products, Daejoo Electronic Materials,Exojet Electronic, Hamilton Precision Metals, Inc., MetalorTechnologies, PEMCO Corporation, Shanghai Daejoo, Young Solar, orZhonglian Solar Technology. The aluminum paste may comprise finealuminum particles dispersed in an organic vehicle. The organic vehiclemay further comprise a binder such as ethyl cellulose or methylcellulose and a solvent such as terpineol or carbitol. Silicon contentis added to the aluminum paste such that the resulting “contact paste”comprises from one to 12 atomic percent silicon.

FIGS. 5A to 5D show that silicon content in the aluminum paste improvesthe formation of the local BSF. The quality of a BSF is defined by theuniformity and thickness of the BSF region. FIGS. 5A to 5D arecross-sectional views from a scanning electron microscope. FIG. 5A is alocal BSF formed from a fritted aluminum paste. FIG. 5B is a local BSFformed from a fritless aluminum paste. FIG. 5C is a local BSF formedfrom a fritless aluminum paste having seven atomic percent silicon. FIG.5D is a local BSF formed from a fritless aluminum paste having 12 atomicpercent silicon. It is evident from FIGS. 5A to 5D that aluminum pasteshaving from one to 12 atomic percent silicon produce higher quality BSFthan aluminum paste having no silicon content. A local BSF may help toachieve a good ohmic contact, especially on a substrate having highresistivity.

Moreover, the local BSF helps to minimize the effect of highrecombination at the metal interface. The back surface recombinationvelocity of an aluminum BSF across the entire back surface isapproximately 500 centimeters per second. In contrast, a dielectric backpassivation with local aluminum BSF formed by an aluminum paste with 12percent silicon reduces the back surface recombination velocity to 125centimeters per second or less.

The contact paste with aluminum and silicon may be applied using ascreen printing machine. For one embodiment of the invention, thecontact paste is fritless. For another embodiment of the invention, thecontact paste is low frit. Fritless or low frit aluminum does not etchor disturb the dielectric layer.

A heat treatment is next applied to the contact paste. In operation 240,the heat is “ramped up” to a temperature from 700 to 900 degreesCelsius. The ramp up time to the peak temperature is from one to fiveseconds. Silicon dissolves into the aluminum at a temperature greaterthan the eutectic temperature, which forms a molten aluminum and siliconalloy. The fast ramp up time helps to form a more uniform BSF. Once thepeak temperature is reached, that temperature is maintained for three orless seconds in operation 250. For example, the peak temperature may bemaintained from one to three seconds. Maintaining the peak temperaturefor this short period of time helps to prevent junction leakage currentbecause there is less chance for impurities to diffuse to the junction.

Finally, the temperature is “ramped down” to 400 degrees Celsius or lessin operation 260. The ramp down time is from three to six seconds. Thisfast ramp down time may be achieved through a forced cool down. Forexample, a fan or a drive belt that removes wafers from the heat sourceat a high speed may be used to rapidly ramp down the temperature to 400degrees Celsius or less.

The fast ramp down provides for passivation in the bulk region. In oneembodiment of the invention, the barrier layer may comprise a hydrogenconcentration from 4×10²¹ to 7×10²² atoms per cubic centimeter. Hydrogenmay be incorporated into the silicon nitride layer by the PECVDprecursors. During the heat treatment, hydrogen may thus bedisassociated from the barrier layer. The hydrogen atoms may then helppassivation in the bulk region of the silicon wafer by attaching todefects in the silicon.

The solubility of silicon in aluminum is proportional to the temperatureof the alloy. Therefore, during cool-down, the percentage of silicon inthe alloy decreases. Excess silicon is rejected from the melt andregrows epitaxially at the silicon liquid interface. This regrowth layergets doped with aluminum according to the finite solid solubility ofaluminum in silicon at the solidification temperature. The regrowthlayer, consequently, becomes a p+ BSF layer.

If pure aluminum is used rather than the aluminum and siliconcombination, the aluminum has an appetite for silicon at hightemperatures. As a result, the rejection of silicon onto the siliconsurface in the openings is decreased. This degrades the quality of rearsurface passivation and lowers the cell performance.

The dielectric layer coupled with the aluminum rear contact havingsilicon also serves to improve absolute cell efficiency. Absolute cellefficiency is measured by a solar cell's ability to convert incominglight into energy. A full area aluminum eutectic back contact has a backsurface reflectance of approximately 60 percent. Back surfacereflectance is defined by the percentage of incident light that isreflected by the back surface back into the silicon. The back contactdisclosed in this invention produces a back surface reflectance ofgreater than 85 percent. The dielectric layer coupled to the aluminumand silicon rear contact improves the cell efficiency by one to twopercent.

The one to 12 atomic percent silicon additive in the contact pasteserves to saturate the aluminum of silicon. Because the aluminum has asilicon concentration, more silicon is rejected from the melt to theopening during cool down. The rejected silicon has an aluminumconcentration and regrows epitaxially at the silicon liquid interfaceforming a p+ BSF layer. Lab tests, the results of which are depicted inFIGS. 5A to 5D, have shown that with the silicon additive, a local BSFdepth from six to 15 micrometers may be achieved.

The rear contact is traditionally applied directly over the entire backsurface of the silicon wafer. If silicon is added to the aluminum pasteand applied to the full back surface of the substrate, then one willobserve a reduction in the BSF layer thickness because less silicon willbe dissolved from the silicon substrate. Thus, it is contrary toconventional wisdom to add silicon to aluminum paste. The inventors,however, have uncovered that the addition of silicon to the aluminumpaste increases the depth of BSF for a local opening geometry. In theabsence of silicon in the aluminum paste, the aluminum layer away fromthe openings needs greater than 12 atomic percent silicon to stay inequilibrium during the cool-down. This reduces the amount of siliconavailable for regrowth in the openings, resulting in thinner local BSF.The addition of silicon to the aluminum paste satisfies the appetite forsilicon in the aluminum. Therefore, most of the silicon in the moltenaluminum-silicon alloy in the openings is available for regrowth,resulting in thicker local BSF.

In addition to improving BSF, the contact paste with silicon may help toprevent aluminum spiking. The solubility of silicon in aluminum rises astemperature increases. As silicon diffuses into the aluminum, thealuminum will in turn fill voids created by the departing silicon. Ifthe aluminum penetrates the p-n or p⁺-p junction of the silicon wafer, alower performance will result.

As discussed above, because the contact paste has from one to 12 atomicpercent silicon, the aluminum will already be saturated with siliconatoms. Thus, silicon atoms from the substrate are prevented fromdiffusing into the aluminum layer during the heat treatment. Aluminumspiking is thereby avoided since no voids will be created in thesubstrate by departing silicon.

FIGS. 6A through 6D depict cross sectional views for one embodiment of asilicon wafer at various stages in the fabrication process. FIG. 6Adepicts a silicon wafer having a doped substrate 600 coupled to adiffused layer 610.

A dielectric layer 620 is coupled to doped substrate 600 in FIG. 6B. Inaddition, a dielectric layer 630 is coupled to diffused layer 610. Thisdielectric layer 620 may be silicon dioxide. The dielectric layer 620may be formed by a spin-on process as described above.

FIG. 6C depicts a barrier layer 640 that is coupled to the dielectriclayer 620 and a barrier layer 650 that is coupled to the dielectriclayer 630. The barrier layers 640 and 650 may be comprised of siliconnitride that is formed by PECVD. The barrier layers 640 and 650 provideprotection to the dielectric layers. Moreover, barrier layer 650 mayprovide an anti-reflective coating to the front surface of the solarcell.

FIG. 6D depicts an opening 625 in the dielectric layer 620 and thebarrier layer 640. An opening 635 may also be formed in dielectric layer630 and barrier layer 650. For one embodiment of the invention, theopening 625 and opening 635 may be formed by applying a solar etch pasteto the dielectric layer and then applying a heat treatment to thedielectric layer. The heat treatment may involve a temperature from 300to 380 degrees Celsius. The heat treatment dissolves the dielectriclayer under the paste, forming an opening to the silicon 810 in thedielectric layer 805 as shown in FIG. 8. FIG. 8 depicts a bottom planview of dielectric layer 805 having opening to the silicon 810. Foranother embodiment of the invention, the opening 625 and opening 635 maybe formed by a laser. For yet another embodiment of the invention, theopening 625 and opening 635 may be formed by a mechanical scribe.

The opening 625 may be in the form of a point or a line. FIG. 7A shows abottom plan view of a barrier layer 740 having openings 725 to thesilicon in a point pattern. Point openings may have a rectangular orcircular shape. FIG. 7B shows a bottom plan view of a barrier layer 740having openings 725 to the silicon in a line pattern.

FIG. 6E depicts a rear contact 660 that is coupled to the dielectriclayer 620, barrier layer 640, and the doped substrate 600 via theopening 625. This rear contact may be comprised of aluminum having fromone to 12 atomic percent silicon. The addition of the silicon in thealuminum provides for a high quality BSF 670 having a depth from six to15 micrometers.

In the forgoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modification and changes may be made theretowithout departure from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than restrictivesense.

1. A method comprising: forming a diffused layer on a doped substrate ofa thin silicon wafer having a thickness from 50 to 200 micrometers,wherein the silicon wafer has a front surface and a back surface;forming a spin-on dielectric layer on the back surface of the siliconwafer; forming a barrier layer on the spin-on dielectric layer; applyingan etch paste from one to 10 percent of the surface area of the barrierlayer; applying a first heat treatment to the etch paste at atemperature from 300 to 380 degrees Celsius; and removing residue froman opening with a solution comprising hydrofluoric acid.
 2. The methodof claim 1, wherein the first heat treatment is applied for a timeperiod from 30 to 45 seconds.
 3. The method of claim 1, furthercomprising: applying a contact paste to the back surface of the siliconwafer following the first heat treatment.
 4. The method of claim 3,wherein the contact paste is an aluminum paste having from one to 12atomic percent silicon.
 5. The method of claim 3, further comprising:applying a second heat treatment to the contact paste at a peaktemperature from 700 to 900 degrees Celsius.
 6. The method of claim 5,wherein the second heat treatment is applied for one to three seconds atthe peak temperature.
 7. The method of claim 1, further comprising:determining etch paste application to portions of the surface area ofthe dielectric layer using a simulations system.
 8. The method of claim7, further comprising: entering parameters into the simulation systemfor determining etch paste application, wherein the parameters comprisean emitter sheet resistance, a cell thickness, a resistivity, a frontsurface recombination velocity, a back surface recombination velocity atthe dielectric, and a contact resistance.
 9. The method of claim 8,wherein the emitter sheet resistance is from 70 to 90 ohms per square,the cell thickness is from 90 to 200 micrometers, the resistivity isfrom 1.5 to 2.5 ohm-centimeter, the front surface recombination velocityis from 50,000 to 70,000 centimeters per second, the back surfacerecombination velocity of the dielectric is from 40 to 60 centimeters,and the contact resistance is zero ohm-centimeter squared.